The present invention relates to a composite electrode including an organic disulfide compound to be used in electrochemical elements such as batteries, electrochromic display devices, sensors, and memories, and also relates to a method of producing the same.
Conductive polymer electrodes have been studied intensively since the discovery of conductive polyacetylene in 1971, because application of conductive polymers for the electrode material may result in desirable electrochemical elements, such as batteries that are light in weight and have a high energy density, electrochromic devices of large areas, and biochemical sensors utilizing microelectrodes.
Since polyacetylene is unstable and not practical as electrodes, other .pi. electron conjugated conductive polymers have been examined. As a result, relatively stable polymers, such as polyaniline, polypyrrole, polyacene, and polythiophene have been found, and lithium secondary batteries utilizing these polymers for the cathode have been developed. An energy density of these batteries is considered to range from 40 to 80 Wh/kg.
Organic disulfide compounds have been proposed in U.S. Pat. No. 4,833,048 as an organic material that can further enhance an energy density. The most simplified form of such compounds is represented by the formula M.sup.+ -.sup.- S-R-S.sup.- -M.sup.+, wherein R denotes an aliphatic or aromatic organic group, S indicates a sulfur atom, and M indicates a proton or metal cation. Such compounds are bonded to each other via S--S bonds by electrolytic oxidation, and form a polymer represented by M.sup.+ -.sup.- S-R-S-S-R-S-S-R-S.sup.- -M.sup.+.
The thus formed polymer is regenerated into the original monomers by electrolytic reduction. A metal-sulfur secondary battery prepared by combining a metal M that supplies and captures cations (M.sup.+) with an organic disulfide compound has been proposed in the U.S. patent mentioned above. This proposed metal-sulfur secondary battery is expected to have an energy density of not less than 150 Wh/kg, which is comparable to or even better than the conventional secondary batteries.
These organic disulfide compounds, however, have a problem; that is, repeated oxidation and reduction (i.e., charging and discharging) results in a gradual decrease of the electrode capacity. Oxidation (charging) of the organic disulfide compound yields a polydisulfide compound which is electrically insulating and has a poor ionic conductivity. The polydisulfide compound has poor solubility in electrolytes.
In contrast, organic disulfide monomers which are generated by reducing (discharging) the polydisulfide compound have high solubility in electrolytes.
Repeated oxidation and reduction accordingly causes part of disulfide monomers to be dissolved in an electrolyte and subsequently polymerized at a position apart from the original position in the electrode. Thus, the polydisulfide compound deposited apart from a conductive agent, such as carbon, is isolated from the electron/ion conductive network in the electrode and does not contribute to the electrode reactions.
Repeated oxidation and reduction increases the isolated polydisulfide compound and gradually lowers the capacity of the battery. The organic disulfide monomers having the high solubility actively move from the cathode to the separator, into the electrolyte, and even further to the anode. The battery with an electrode including an organic disulfide compound as the cathode accordingly has drawbacks, such as lowered charging and discharging efficiency and a shortened charge and discharge cycle life.